Transparent membranes for gas lenses and light guidance system employing same

ABSTRACT

Optical elements are formed of static gases of various refractive indices separated by optically thin transparent membranes, such as collodion or aluminum oxide films. Lenses are described having flat, approximately cylindrical and approximately spherical surfaces, and formulas are provided which are useful for the designing of a light guidance system using these lenses.

United State:

Berreman I151 3,652,150 1 51 Mar. 28, 1972 i541 TRANSPARENT MEMBRANESFOR GAS LENSES AND LIGHT GUIDANCE SYSTEM EMPLOYING SAME [72] inventor:Dwight W. Berreman, Westfield, NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

22 Filed: July 1,1968

[2i] Appl.No.: 741,449

[52] U.S. CL ..350/l79, 350/96, 350/286 [51] Int. Cl. ..G02b l/06 [58]Field oISearch ..350/179, 180, 175,96

[56] References Cited UNITED STATES PATENTS 504,890 9/1893 Ohmart..350/l79 UX 2,300,251 10/1942 Flint ..350/ l 80 3,169,163 2/1965Nassenstein ..350/l75 GN UX 3,382,022 5/1968 Fox ..3$0/96 X 3,454,3307/1969 Smith ..350/287 OTHER PUBLICATIONS Hauser et al., Easily MountedAluminum Oxide Foils for Windows and Backings" The Review of Scientificinstruments, Vol.29, No.5, May 1958, pp. 380- 382.

Primary Examiner-John K. Corbin Attorney-R. J. Guenther and Edwin B.Cave [5 7 1 ABSTRACT Optical elements are formed of static gases ofvarious refractive indices separated by optically thin transparentmembranes, such as collodion or aluminum oxide films. Lenses aredescribed having flat, approximately cylindrical and approximatelyspherical surfaces, and formulas are provided which are useful for thedesigning of a light guidance system using these lenses.

18 Claims, 4 Drawing Figures PAIENTEnmza I972 FIG...

FIG.

FIG. 4

. IN VENTOR By 0. w. BERREMAN AT RNEV TRANSPARENT MEMBRANES FOR GASLENSES AND LIGHT GUIDANCE SYSTEM EMPLOYING SAME BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to the longdistance transmission of beams of electromagnetic wave energy, includingvisible light and adjacent frequencies within the wavelength range offrom 0.1 microns to 200 microns and to the prevention of scattering ofthe rays of such beams, during transmission.

2. Prior Art Laser generated highly directive beams of the coherent,high frequency electromagnetic wave energy, principally in the visibleand adjacent energy bands, having wavelengths from about 1,000 to 2million angstroms, are recognized as having extremely large capabilitiesfor the long distance transmission of intelligence such as speech,video, data signals, etc.

However, transmission of these beams over substantial distances isaccompanied by a very appreciable spreading of the beam, i.e.,attenuation, resulting in a significant diminution of the energyreceived at a distant point on the axis of the beam, and also resultingin the possibility of interception of portions of the beam by stationsother than the intended receiving station.

Enclosing the beam in a conduit filled with a transmission medium ofuniform characteristics, so as to eliminate the effects of a changingatmospheric condition and the possibility of beam interception, mayresult in the multiple reflection of the spreading rays by the conduitwalls, seriously distorting the transmitted signals.

It has been recognized that beam spreading may be substantiallyeliminated by the use of a series of positive lenses along the beam pathto divert the outer rays toward the central axis of the path. it hasalso been recognized that many of the problems associated with the useof glass lenses can be overcome if instead a very low index focusingmedium such as a gas is used. For example, the use of gas lensespractically eliminates reflection losses at the lens surfaces.

in a practical guidance system, however, in which a large number oflenses are distributed at great distances from one another, it is likelythat such lenses will be slightly and randomly displaced from coaxiallydistributed positions, thus causing the beam to wander," i.e., tooscillate about the axis of the intended transmission path. Since somebeam wander is practically unavoidable, large aperture lenses aredesirable to insure that significant portions of the beam are not lostfrom the transmission path. Present gas lenses depend upon substantiallylaminar flow of the gases for their operation, whether such flow is byforced convection, thermal convection or other means. Thus, the lensdiameters, and consequently the lens apertures, are limited by the onsetof turbulent fiow at critical flow velocities, which are very smallunless diameters are very small.

SUMMARY OF THE INVENTION The problems associated with glass lens andmoving-gas lens guidance systems are avoided by the employment of staticgases of differing refractive indices, separated by optically thintransparent membranes, as optical elements.

According to the invention, reflection losses at the membrane surfacesare minimized by maintaining their thicknesses at either a smallfraction of the wavelength of the beam radiation, or at an integralmultiple of one-half the wavelength of the radiation.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of astatic gas lens having cylindrical surfaces, according to the invention;

FIG. 2 is a cross sectional view of a static gas lens having a sphericalsurface, according to the invention;

FIG. 3 is a perspective view of a static gas prism according to theinvention; and

FIG. 4 is a schematic diagram of a guidance system using the lenses ofthe invention.

DETAILED osscarmorv The membrane may be any solid material which can beformed into optically thin sheets of sufficient size. mechanicalstrength, gas-imperviousness and transparency for the use contemplated.

An excellent example of such a material is A50 Preparation of films ofAl,0, is described in the art, and thus a detailed description of themis considered unnecessary. A detailed procedure involving anodlzstion ofaluminum to form the oxide film, followed by dissolution of thealuminum, described by l-lauser et al. in the "Review of ScientificInstruments," 29, 380 (1958), results in gas-tight, smooth, transparent,pressure resistant films, of easily controllable and uniform thicknessof from 700 to 7,000 angstroms. Such a procedure is desirable in that itenables the formation of films in a variety of configurations such assheets or sphericallyshaped films, by means of shaping the surface ofthe aluminum to be anodized, to correspond to the desired shape of thefilm; and in that it enables the formation of a supporting frame bymeans of selectively dissolving the aluminum subsequent to anodization.Aluminum oxide films having thicknesses as small-as 200 angstroms havebeen obtained in a similar way.

For ease of fabrication of lenses, it may be preferred to form fiatsheets of the membrane of sufficient thinness to permit formation ofcylindrical surfaces by bending. FIG. 1 depicts a lens having a frame 10shaped so that two cylindrical membrane surfaces 11 and 12 are formedwhich have their axes at right angles. Such a lens focuses radiationpassing through it much as if the surfaces were spherical.

For a given film material of given thickness, it should be noted thatthe amount of overpressure that can be applied without rupture increasesas the radius of curvature decreases.

Elastic materials may be preferred over rigid materials such as Al,0,where approximately spherical surfaces are desired, since such surfacesmay be formed by maintaining an overpressure on the outside of a flatelastic membrane, so as to force it to expand slightly. Such a lens isexemplified by the configuration depicted in FIG. 2, wherein frame 20and membranes 22 and 23 form chamber 21, which contains a gas at alesser pressure than a gas outside chamber 21.

One such elastic material suitable for use in such lenses is known ascollodion, and is composed of nitrocellulose carried in a volatilesolvent such as ether, alcohol, amyl acetate or materials such ascollodion to some gases may necessitate aslow flushing of the gases,particularly in the low pressure side, to maintain a constant differencein refractive index across the membrane. In particular, it may bedesirable to avoid the use of gases which have large diffusionconstants, such as He and Ar, in order to avoid the necessity forrecycling. Alternatively, lenses having concave surfaces and theinterior gas of smaller refractive index than the surrounding gas and ata slight underpressure may be desirable in that most of thecontamination due to diffusion would be in the small volume of the lens,thus minimizing the necessity for flushing the larger volume ofsurrounding gas.

Of course it is possible and will often be advantageous to mix differentgases to give any desired refractive index or density. For example,where very thin elastic membranes are employed, it may be desirable tochoose gases which while having different refractive indices, have aboutthe same densities, so as to minimize aberrations due to a gravityeffect. One example of a pair of such gases would be air, having anindex of 1.00029 and a density of 0.00129 grams per cubic centimeter,and ethane, having an index of 1.00075, and a density of 0.00125 gramsper cubic centimeter, all at 0 C. and l atmosphere.

The thickness of the membrane desired will in general depend upon thewavelength of the radiation, the me and strength of the membranedesired, the permeability of the membrane and the ease with whichuniformity of thickness is attained. in order to minimize reflectionlosses at the membrane surfaces, its thickness should be a smallfraction of the wavelength of the radiation or an integral multiple ofone-half the wavelength inside the solid membrane medium. The wavelengthinside the membrane is detennined by the following relation, in which nis the refractive index of the membrane medium and A, is the radiationwavelength in vacuum; A inside I h ln, for Al,O,, n is about 1.65 forvisible light.

In general, it can be said that where the thickness of the membrane isless than about one-tenth the wavelength of the radiation, thereflection loss is negligible. Thus, when a material is chosen which isdifficult to form into membranes of unifonn thickness, a thicknesssmaller than one-half the wavelength may be preferred to ease thetolerance on thickness. It will be appreciated that one advantageinherent in the use of focusing media such as gases, giving rise tosmall differences in refractive index across the lens surface, is thesmall loss due to surface irregularities, thus resulting in a largesurface configuration tolerance for the membranes, as compared tofocusing media having large differences in refractive index, such asglass lenses in air.

The physical principles upon which the invention is based give rise toseveral relationships which are useful in the design of a light guidancesystem using the lenses described. As will be seen from theserelationships, the adjustment of the various parameters involved permitsgreat flexibility in the design of a practical system.

For purposes of illustration only, the case for transmission along astraight line path will be treated. Most convenient for this purpose isthe distribution of positive lenses of equal strengths at equalseparations along the transmission path as shown schematically in H0. 4.In the figure, 40 and 41 represent lenses, 42 and 43 define theboundaries of the Gaussian beam mode diameter, and 44 represents thecentral axis of the intended transmission path. Such a configurationresults in the repeated slight convergence of the outer rays of a lightbeam toward axis 44 at each lens, so as to substantially eliminate "beamspreading." The somewhat arbitrary choice of a lens separationequivalent to the lens focal length was likewise chosen for convenience.However, it should be noted that for lens separations larger than fourfocal lengths, the lens system will not confine the beam, and for lensseparations much smaller than the focal length, the number of lenses maybe economically undesirable.

It may be shown for lenses separated by their focal length, that 5.45b x(1) and that R: ("i 1) (O a) gwhich describes an easily obtainable shapefor any size lens; )t 10,000 A. or cm. (near infrared light); and

Equation (5) results in a system which allows beam wander up to adistance R/2 with little light loss. The remaining parameters may bedescribed as dependent upon those set forth above.

Combining equations (2) and (3) results in Typical results are obtainedby choosing carbon dioxide as the medium inside the lens and air as themedium outside the lens, which mediums have refractive indices equal to1.000405 and l.000266 respectively. From equation (9), therefore, theradius of the lens opening R is approximately equal to 7.2 centimeters.From equation (6) the focal length I is approximately equal to 7.8 X 10centimeters, which is approximately equal to one-half mile.

Where the medium inside the lens is chosen to be one-half carbon dioxideand onehalf air and the medium outside the lens remains air, the radiusof the lens opening is doubled and the focal length is increased by afactor of 4.

The above results are illustrative only and apply only to straight linepropagation. For example, where the transmission path is intended to becurved, the radius of the lens opening should be many times larger orprisms should be used, as is known in the art. Accordingly, a variety oflens configurations and combinations, as well as other optical elements,such as the prism depicted in FIG. 3, wherein frame 30 supports flatmembranes 31 and 32, will become obvious to those skilled in the art,and are intended as being encompassed within the scope of the inventionand the appended claims.

What is claimed is: l. A light guidance system for use in the longdistance transmission of beams of coherent monochromatic radiationthrough a gaseous transmission medium, said system comprising:

a plurality of optical elements, at least a portion of these elementsbeing substantially coaxially distributed,

characterized in that each of said elements has at least two transparentsurfaces, at least one gas having a refractive index different from thatof said transmission medium and at least two transparent membranesfonning each of said transparent surfaces and separating said gas fromsaid medium,

and further characterized in that said membranes have a thickness of upto one-tenth the wavelength of the radiation in said membranes.

2. The elements of claim 1 in which at least a portion of said membraneis approximately cylindrically shaped so as to result in positivefocusing of a beam of radiation passing through said element.

3. The element of claim 1 in which at least a portion of said membraneis approximately spherically shaped so as to result in positive focusingof a beam of radiation passing through said element.

4. The element of claim 1 in which said membrane comprises at least oneflat surface, said surface being slanted with relation to an incidentbeam of radiation so as to result in refraction of said beam passingthrough said element.

5. The element of claim 1 in which said membrane is Al,0,.

6. The system of claim 1 in which at least a portion of said elementsare positive lenses distributed substantially colinearly along thecentral axis of a light beam transmission path, so as to result in theslight convergence of the outer rays of a light beam toward said centralaxis at each lens.

7. The system of claim 6 in which said positive lenses are concave andcontain gases having refractive indices smaller than the refractiveindex of the surrounding medium.

8. The system of claim 6 in which said positive lenses are convex andcontain gases having refractive indices larger than the refractive indexof the surrounding medium.

9. The element of claim 1 in which said membrane is collodion.

10. A light guidance system for use in the long distance transmission ofbeams of coherent monochromatic radiation through a gaseous transmissionmedium, said system comprismg:

a plurality of optical elements, at least a portion of these elementsbeing substantially coaxially distributed, characterized in that each ofsaid elements has at least two transparent surfaces, at least one gashaving a refractive index different from that of said transmissionmedium and at least two transparent membranes forming each of saidtransparent surfaces and separating said gas from said medium,

and further characterized in that said membranes have a thickness equalto an integral multiple of one-half the wavelength of the radiation insaid membranes.

11. The elements of claim 10 in which at least a portion of saidmembrane is approximately cylindrically shaped so as to result inpositive focusing of a beam of radiation passing through said element.

12. The element of claim 10 in which at least a portion of said membraneis approximately spherically shaped so as to result in positive focusingof a beam of radiation passing through said element.

13. The element of claim 10 in which said membrane comprises at leastone flat surface, said surface being slanted with relation to anincident beam of radiation so as to result in refraction of said beampassing through said element.

14. The element of claim 10 in which said membrane is Al,o,.

15. The system of claim 10 in which at least a portion of said elementsare positive lenses distributed substantially colinearly along thecentral axis of a light beam transmission path, so as to result in theslight convergence of the outer rays of a light beam toward said centralaxis at each lens.

16. The system of claim 10 in which said positive lenses are concave andcontain gases having refractive indices smaller than the refractiveindex of the surrounding medium.

17. The system of claim 10 in which said positive lenses are convex andcontain gases having refractive indices larger than the refractive indexof the surrounding medium.

18. The element of claim 10 in which said membrane is collodion.

l t i i i

1. A light guidance system for use in the long distance transmission ofbeams of coherent monochromatic radiation through a gaseous transmissionmedium, said system comprising: a plurality of optical elements, atleast a portion of these elements being substantially coaxiallydistributed, characterized in that each of said elements has at leasttwo transparent surfaces, at least one gas having a refractive indexdifferent from that of said transmission medium and at least twotransparent membranes forming each of said transparent surfaces andseparating said gas from said medium, and further characterized in thatsaid membranes have a thickness of up to one-tenth the wavelength of theradiation in said membranes.
 2. The elements of claim 1 in which atleast a portion of said membrane is approximately cylindrically shapedso as to result in positive focusing of a beam of radiation passingthrough said element.
 3. The element of claim 1 in which at least aportion of said membrane is approximately spherically shaped so as toresult in positive focusing of a beam of radiation passing through saidelement.
 4. The element of claim 1 in which said membrane comprises atlEast one flat surface, said surface being slanted with relation to anincident beam of radiation so as to result in refraction of said beampassing through said element.
 5. The element of claim 1 in which saidmembrane is Al2O3.
 6. The system of claim 1 in which at least a portionof said elements are positive lenses distributed substantiallycolinearly along the central axis of a light beam transmission path, soas to result in the slight convergence of the outer rays of a light beamtoward said central axis at each lens.
 7. The system of claim 6 in whichsaid positive lenses are concave and contain gases having refractiveindices smaller than the refractive index of the surrounding medium. 8.The system of claim 6 in which said positive lenses are convex andcontain gases having refractive indices larger than the refractive indexof the surrounding medium.
 9. The element of claim 1 in which saidmembrane is collodion.
 10. A light guidance system for use in the longdistance transmission of beams of coherent monochromatic radiationthrough a gaseous transmission medium, said system comprising: aplurality of optical elements, at least a portion of these elementsbeing substantially coaxially distributed, characterized in that each ofsaid elements has at least two transparent surfaces, at least one gashaving a refractive index different from that of said transmissionmedium and at least two transparent membranes forming each of saidtransparent surfaces and separating said gas from said medium, andfurther characterized in that said membranes have a thickness equal toan integral multiple of one-half the wavelength of the radiation in saidmembranes.
 11. The elements of claim 10 in which at least a portion ofsaid membrane is approximately cylindrically shaped so as to result inpositive focusing of a beam of radiation passing through said element.12. The element of claim 10 in which at least a portion of said membraneis approximately spherically shaped so as to result in positive focusingof a beam of radiation passing through said element.
 13. The element ofclaim 10 in which said membrane comprises at least one flat surface,said surface being slanted with relation to an incident beam ofradiation so as to result in refraction of said beam passing throughsaid element.
 14. The element of claim 10 in which said membrane isAl2O3.
 15. The system of claim 10 in which at least a portion of saidelements are positive lenses distributed substantially colinearly alongthe central axis of a light beam transmission path, so as to result inthe slight convergence of the outer rays of a light beam toward saidcentral axis at each lens.
 16. The system of claim 10 in which saidpositive lenses are concave and contain gases having refractive indicessmaller than the refractive index of the surrounding medium.
 17. Thesystem of claim 10 in which said positive lenses are convex and containgases having refractive indices larger than the refractive index of thesurrounding medium.
 18. The element of claim 10 in which said membraneis collodion.